How to rearrange poses in Pose Libraries?
Technically you can but I will say it's not worth the effort (at least with the current version). This could be an idea for a feature request or a possible addon.

To briefly cover the details for the adventurous, a pose library is nothing more than an action saved for the armature. The pose library name is the action name. The first pose is saved at frame one, the second at frame two and so on. By going to the action editor (dopesheet - action editor context) you can re-arrange the order of the saved poses. This means that the pose names will no-longer match up but you could then rename them to match. Once you start you need to keep track of what frame is for what pose name. If you have 5 poses to re-arrange it's not too hard otherwise it's easier to leave it for now.